Introduced a feature to conveniently link the metallicity to the iron abundance by

- adding a new function `link_metallicity_to_iron',
- adding new variables `Z0_abundances_1' and `Z0_abundances_2' to the "spectrum_fit" namespace in `my_isisscripts.sl',
- updating the templates `make_fit_grid.sl', `spectroscopy_double_star.sl', `spectroscopy_single_star.sl',
- updating the functions `all_syn_spec_ranges' and `initialize_grid_fit'.